Our laboratory has developed a coordinated research effort focused on the study of regulation of cellular proliferation and differentiation using the hematopoietic system as a model. The basis of hematopoietic regulation appears to cell-cell interaction, whether mediated via direct cell-cell contact or over short distances through soluble growth factors. We have recently described a series of studies showing that tumor promoting phorbol esters have dramatic effects on proliferation and differentiation of hematopoietic progenitor cells in vitro. Active phorbol esters can apparently replace glycoprotein colony stimulating factor in the stimulation of mouse marrow granulocyte-macrophage colony formation in vitro. These are the first chemically and structurally defined substances which can substitute for colony stimulating factor in this system. We have also recently described potent effects of phorbol esters on murine erythropoiesis in vitro. The structure-activity relationships which have been defined for in vivo tumor promotion appear to hold as well for effects on in vitro hematopoiesis. We plan to further investigate the mechanisms of action of tumor promoting phorbol esters on murine hematopoiesis. We will study their effects on development of pluripotent stem cells and megakaryocytopoiesis as well as on recently defined cellular regulatory systems which operate in vivo and in vitro to control erythropoiesis. We will further examine the effects of phorbol esters on hematopoiesis in long term bone marrow cultures and examine the role of the extracellular matrix in such cultures. We will systematically assay effects of phorbol esters on normal human hematopoiesis in vitro using clonal assays and long-term bone marrow culture. Additionally, we will also study effects of systemic administration of phorbol derivatives to mice with special attention to effects on the hematopoietic system. Since clinical disorders of the hematopoietic system, such as the lymphohematopoietic malignancies, are characterized by failure of regulatory control over proliferation and differentiation, a better understanding of hematopoietic regulation will contribute to our management of these disorders.